1. Field of the Invention
This invention relates to apparatus and process for printing large graphics, such as billboards, outside advertising matter, signs and displays of various kinds, such as used in public areas and the like. More particularly, it relates to such an apparatus and a process which is adapted to reproduce on a large scale any desired colored signs and or pictures on a flexible substrate, such as paper.
2. The Prior Art
While the graphic art is very highly developed and has achieved high quality technical results, the problem of printing large graphics and obtaining good results rapidly and economically has not found a completely satisfactory solution. Photographic techniques, such as those commonly used for small reproductions, are not adapted for large ones and, at any rate, are excessively expensive. Painting by hand, which is a method still resorted to in many cases, is obviously time-consuming and expensive.
Apparatus for printing large graphics has been described in the prior art. For instance,
U.S. Pat. No. 3,553,371 discloses a method and device for enlarging multicolor printing, by scanning an original colored picture; separation of light beams corresponding to each spot of the original picture into primary colors, converting the light beam into electric signals representing density of each primary color, actuating spray guns, each assigned to a primary color, responsive to said electric signals, with proper time delay, so that a spot on a single printing medium, which corresponds to said scanned point of the original picture, is printed in an extremely enlarged dimension by said spray guns in multicolor.
U.S. Pat. No. 4,547,786 discloses an apparatus in which a plurality of vertical panels are passed in a closed path, in a succession of continuous revolutions, in front of an ink-jet printing station which is displaceable in a vertical direction by incremental displacements, amounting, e.g., to one-tenth of an inch per revolution of the panels. The printing station, when printing in colors, comprises several groups of printing heads, each color being provided by one of said groups, the different printing head groups being spaced vertically from one another, so that, once a color has been fed onto a horizontal line of a panel, the next color will be fed thereonto only after a certain number of revolutions of the panel. The incremental displacements of the printing station are therefore slaved to the revolutions of the panels. An advertising sign or billboard or the like may be made by joining several panels.
U.S. Pat. No. 4,811,038 discloses an apparatus which comprises a drum which revolves about a horizontal axis, onto which the print backing may be applied, and a printing station which is displaceable horizontally and comprises vertically spaced printing head groups for feeding the different colors. The printing station moves incrementally in a horizontal direction by a given distance for each revolution of the drum, and once a color has been printed on a peripheral circle of the print backing applied on the drum, a certain number of revolutions, e.g. twenty, of the drum will have to take place before the same peripheral circle is imprinted with the next color. The incremental displacements of the printing station are therefore slaved to the revolutions of the drum.
The apparatus of the aforesaid patents are not wholly satisfactory, for a number of reasons. They are mechanically cumbersome and slow. The printing station "scans" straight lines or circles on the material to be printed, but the scanning is produced by keeping the printing station still, during each scanning, and moving it incrementally between successive scannings, so that in fact the scanning motion is that of the print backing, which involves the motion of fairly cumbersome mechanical elements. Each scanning requires in all cases at least a complete traversing of the breadth of the print backing. Secondly, the accurate registration of the various colors requires extreme precision in the mechanical movements. If twenty revolutions of the panels of the first mentioned U.S. patent or twenty revolutions of the drum of the second mentioned U.S. patent must occur between feeding two different colors on the same scan line, it is clear that any play or inaccuracy in the mechanical movements which occurs during a revolution is multiplied and the registration of the different colors is made difficult and dependent on extreme mechanical accuracy, which in turn requires expensive apparatus. Although the mass of the print backing may be small, that of the mechanical parts which actuate the backing is considerable, and their speed must be contained so as to avoid vibrations and deformations. The length of the sheets, or other print backing that can be used, is limited, such as the number of panels, in U.S. Pat. No. 4,547,786, or the radius of the drum, in U.S. Pat. Nos. 3,553,371 and 4,811,038, are limited. Further, with the apparatus described in the aforementioned U.S. patents, the number of points or ink drops per unit area of the pixels is limited, e.g., as described in U.S. Pat. Nos. 4,547,786 and 4,811,038, if the pixels are squares of one-tenth of an inch size, each of them will receive nine ink drops, viz. can be represented by a 3.times.3 matrix.
It is a purpose of the present invention to provide an apparatus and process for printing large graphics, which are free of the aforementioned drawbacks, and are mechanically simple and economical and yet provide excellent graphic results.
It is another object of the present invention to provide such an apparatus and process which can operate at high speed and are economical.
It is a further object of the present invention to provide such an apparatus and process in which there are ideally no limits to the length of the material to be printed.
It is a still further purpose of the invention to provide such an apparatus and process which solves all the problems of the registration of the various colors.
It is a still further purpose of the invention to provide an apparatus and process whereby inaccuracies and plays in the motions of the mechanical parts which carry the print backing or substrate have no substantial influence on the registration of the various colors.
It is a still further purpose of the present invention to provide an apparatus and process which permits to obtain printed graphics that can be viewed, both at a great distance and at a relatively short distance, with results equally pleasing to the eye.
It is a still further purpose of the invention to provide an apparatus and process which permits to obtain high quality half-tone prints.
Other purposes and advantages of this invention will appear as the description proceeds.